In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to move toward a core of the cable, and, if not controlled, to move longitudinally into splice closures. There are some splice closures available commercially in which the cable jacket is terminated inside the closure. Hence, if water is able to travel longitudinally along the cable, it could enter the splice closure, possibly causing a degradation in transmission.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment may cause problems and should be prevented. Also, in some climates, the development of ice within an optical fiber cable may have a crushing influence on the optical fibers in the core which may affect adversely the attenuation thereof.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and into the core. For example, a metallic shield which often times is used to protect a cable against electromagnetic interference is provided with a sealed longitudinal seam. However, because lightning strikes may cause holes in the metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core.
Filling materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material causes housekeeping problems, inhibits manufacturing line speeds because of the need to fill carefully interstices of the core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core. In other arrangements, an atactic material may be used to flood the outer surface of a metallic shield.
Presently, many commercially available cables also include a water-swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two cellulosic tissues. Further included may be a polyester scrim which is used to provide tensile strength for the laminated tape. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware. The problem of tape thickness has been overcome by a water blockable member comprising a substrate tape which has been impregnated with a superabsorbent material. Such a water blockable member is disclosed and claimed in copending application Ser. No. 07/115,123 filed of even date herewith in the name of C. J. Arroyo.
Another factor that must be considered with respect to a water blocking system for a cable is the bonding of the plastic cable jacket to an underlying metallic shield. Where such adhesion is important to the performance of the cable, care must be taken not to interpose a water blocking member therebetween which would impair the desired adhesion.
In the past, it has been commonplace to dispose a water blocking tape between a shield of the sheath system and the core or core tube, if any. Although the use of a laminated water blocking tape or a water blockable tape such as that disclosed in previously identified C. J. Arroyo application Ser. No. 07/115,123 is placed typically between a metallic shield and a core tube, such an arrangement does not prevent the longitudinal flow of water between other elements of the sheath system and into closures, for example.
Seemingly, the prior art does not disclose a cable which is provided with a system which prevents substantially the flow of water longitudinally along the cable sheath system. What is needed and what does not appear to be available in the marketplace is a cable water blocking system which is relatively inexpensive and which does not compromise any desired bonding between members of the cable sheath system. Such a system should be one which is easily provided during the cable manufacturing process.